Video transcript

In this video, I wanna talk about how neurotransmitters are removed from the synapse. In other videos, we've talked about how action potentials travel down axons, and the information that's contained in action potentials, is really contained in the frequency of action potential firing, and the duration of a
train of action potentials. And when the action potential
reaches the axon terminal, at the end of the axon, neurotransmitters are released
into the synaptic cleft to bind to neurotransmitter recepters on the target cell. But if a neuron is
firing action potentials very frequently, if there are large numbers
of action potentials reaching the axon terminal, then the rate of neurotransmitter release into the synapse, may exceed the rate that neurotransmitter can just passively diffuse
out of the synapse, so that diffusion is the first method by which neurotransmitter can be removed from a synapse, "Diffusion." But, that only works if the
neuron is firing action potentials at a slow frequency. At a fast frequency,
diffusion won't be enough, and there'll be a build up of neurotransmitter in the synapse. And this would be a problem, because if the neurotransmitter
is just lingering in the synapse, then
neurotransmitters bound the neurotransmitter
receptor, most of the time, and the information
contained in the frequency, and the duration of trains
of action potentials, wont' be able to be
transmitted to the target cell. The synapse will basically
not be functional, to communicate additional information. Therefore, neurotransmitter may need to be actively removed, instead of just through passive diffusion, to
clear out the neurotransmitter from the synaptic cleft. And it turns out that
there are several ways that this happens. The first of these active methods, or the second method to
remove neurotransmitter from the synapse, are
enzymes that can break down the neurotransmitter in the synapse. So certain synapses contain
enzymes that'll actually break down the neurotransmitter
into its component parts, which are no longer able to stimulate the neurotransmitter receptor. So they're removing
active neurotransmitter from the synapse. The next active method, is that
some pre-synaptic membranes contain special pumps, special
active transport channels, that actively pump back
in the neurotransmitter, into the axon terminal, where it's often recycled, to
be used for a subsequent round of neurotransmission, by being released back into the synapse. So these pumps are
called "Re-uptake Pumps," because they take the
neurotransmitter back into the axon terminal where it came from in the first place. By doing so, they remove
the neurotransmitter from the synaptic cleft. Another big method of actively
removing neurotransmitter from the synapse, is
by "Astrocyte Endfeet." So the astrocytes in the
central nervous system put their endfeet on lots of the synapses in the central nervous system, and they also have pumps
at a lot of these synapses, that can actively pump
the neurotransmitter out of the synapse, into the astrocyte. And sometimes it'll just be broken down, or used in the astrocyte,
or sometimes the astrocyte will actually transfer
some of the substances of the neurotransmitters
back into the axon terminal of the neuron, where it will
be recycled, and used again for neurotransmission. So all of these different
methods allow the synapse to basically be rapidly turned on and off. 'Cause neurotransmitter
can be rapidly released into the synaptic cleft, and then it can be rapidly cleared out, so that the synapse is capable
of conveying more information from the neuron, to the target cell.